The long-term strength of rocks under high-temperature and high-pressure conditions is crucial for assessing the stability of high-level radioactive waste disposal facilities. This study investigates the thermomechanical behavior of Beishan granite, a candidate host rock for Chinese high-level radioactive waste repository, under coupled thermal–mechanical loading. A series of triaxial direct shear creep tests integrated with acoustic emission monitoring were conducted to analyze the damage evolution and failure mechanisms of the rock. Results reveal that as temperature increases from 30 °C to 150 °C, the proportion of tensile failure in the granite increases from 29.03 % to 51.36 %. A thermo-mechanically coupled viscoelastic damage constitutive model is developed to accurately capture the time-dependent mechanical response of granite under triaxial direct shear. After validation against experimental data, the model is implemented into a finite element framework via a user subroutine to predict the long-term stability of an high-level radioactive waste disposal unit. Furthermore, fuzzy set theory was applied to optimize the layout parameters of the disposal unit, leading to an enhanced safety and efficiency profile; the optimal layout is determined with a canister number (NC) to canister spacing (SC) to unit spacing (SI) ratio of 1:3:10. These results provide valuable insights into the thermo-mechanical behavior of granite and offer a practical tool supporting the design and safety evaluation of deep geological repositories, thereby facilitating the sustainable development of nuclear energy.
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